The present invention generally relates to emergency power units on aircraft. More specifically, the present invention relates to an improved emergency power unit having a JP-air subsystem which can operate with a bleed air subsystem for the purpose of minimizing turbine underspeed operation during transition from bleed air mode to JP-air mode. This is accomplished by reducing the time required to ignite the mixture in a main combustor of the JP-air subsystem and then increasing combustor output to the level required to develop the original speed.
An aircraft generally has one or more primary engines that provide thrust for the aircraft, as well as pressurized bleed air for the environmental control system. The primary engine also provides power to drive electric generators and hydraulic pumps, both of which are necessary for powering instruments and flight control systems. In addition, many aircraft also have an auxiliary power unit (APU) to provide power to the aircraft either on the ground or in flight, or both. This power may be provided in the form of one or more of electrical power, hydraulic power, and pressurized air according to the requirements of the aircraft. Unfortunately, starting of an APU may require from many seconds to as much as a few minutes. During this starting time, power from the APU is, of course, not available to the aircraft. As a result, some essential aircraft systems may not be operated during starting of the APU. Also, if the aircraft is above a determined altitude, it may not be possible to start the APU because of low ambient pressure. An aircraft with only an APU may require some other system, such as an emergency power unit (EPU), to provide power to the aircraft until the APU can be started.
The EPU is employed to provide hydraulic or electric power (or both) on a relatively short term basis after a failure of an essential system associated with the aircraft main engines. This emergency power supply allows continuation of controlled aircraft flight for a limited time while the aircraft is brought to a landing or to an altitude low enough to allow starting of the aircraft APU or restarting the main propulsion engine. The development of unstable aircraft has increased the need for providing a rapidly available source of emergency power. Upon a failure of the main hydraulic pump, or main generator, or of the aircraft propulsion engine driving these devices, the aircraft cannot be maintained in controlled flight. Without hydraulic power to move the aircraft control surfaces, or electrical power for flight control computers, the unstable aircraft is uncontrollable. Thus, these aircraft must have a source of emergency power which is available almost immediately after the failure of a flight control related power system.
In the past, EPUs have employed a hydrazine decomposition chamber or a jet fuel (e.g., JP fuel) combustor to provide a flow of high temperature pressurized motive gas to a turbine. In turn, the turbine drives a hydraulic pump or electric generator, for example. One example of a hydrazine chamber is shown in U.S. Pat. No. 4,554,784. Typically, liquid hydrazine has been sprayed onto a catalyst bed. The amount sprayed is metered to control the volume of gas produced by the chamber which then controls the speed of the turbine. However, this system has disadvantages, including the fact that hydrazine is corrosive and toxic. In order to undergo decomposition, the hydrazine must be sufficiently unstable, which presents a safety hazard. When the catalyst is depleted, the catalyst must be replaced and is oftentimes expensive. Further, hydrazine and appropriate storage facilities for it may not be readily available at different locations.
A jet fuel combustor is shown, for example, in U.S. Pat. No. 4,898,000. Within a combustion chamber, air from a tank is mixed from a separate tank. The fuel rich mixture is then ignited to drive a turbine. An advantage of the jet fuel combustor over the hydrazine chamber is that jet fuel, such as JP fuel, is more readily available and non-toxic. However, a limitation is that it can often take up to about 5 seconds to ignite the system. This length of delay could likely be enough time for a pilot to lose control of the aircraft. Further, a jet fuel combustor (as well as the hydrazine chamber) has a limited run time.
In an effort to overcome at least some of the above design limitations, a jet fuel combustor subsystem has been combined with a bleed air subsystem. This is shown, for example, in U.S. Pat. No. 4,898,000. In general, the combination of a jet fuel combustor subsystem and bleed air subsystem enables the subsystems to be alternately operated, thereby increasing the total run time of the EPU. Unfortunately, as the subsystems are alternately started and stopped, there is a turbine underspeed for a period of time during which the turbine speed drops due to a lack of power input. The drop time can vary, for example, from 1 to 5 seconds. While seemingly of short duration, the effects can be significant. The time during which the turbine speed drops below a required speed means that hydraulic and/or electrical power is likewise reduced during such time. But the reduction of hydraulic and/or electrical power may be of sufficient duration to cause a loss of control of the aircraft.
As can be seen, there is a need for an improved EPU, including one that can be used on aircraft. Another need is for an improved EPU that incorporates both a jet fuel combustor subsystem and bleed air subsystem. Also needed is an EPU that has, in effect, a decreased starting time. Similarly, an EPU is needed that reduces the turbine speed drop time that can occur when alternating operation from a bleed air subsystem and to a jet fuel combustor subsystem.